Among other things, piezoelectric elements are used in fuel injectors for internal combustion engines. If, for example, the piezoelectric element is used as an actuator in a fuel injection system, it is necessary for certain applications that the piezoelectric element be able to be brought to different expansions or if needed to varying expansions as precisely as possible. Through direct or indirect transmission to a control valve, different expansions of the piezoelectric element correspond to the displacement of an actuator, like a nozzle needle for example. The displacement of the nozzle needle results in the opening of injection orifices. The duration of the opening of the injection orifices corresponds to a desired injected fuel quantity as a function of a free cross section of the orifices and an applied pressure.
The transmission of the expansion of the piezoelectric element to the control valve is differentiated here into two basic transmission modes. In the first, direct, transmission mode, the nozzle needle is moved directly by the piezoelectric element via a hydraulic coupler. In the second transmission mode, the movement of the nozzle needle is controlled by a control valve which is triggered by the piezoelectric element via a hydraulic coupler. The hydraulic coupler has two characteristics: first, the reinforcement of the stroke of the piezoelectric element, and second, the decoupling of the movement of the control valve and/or the nozzle needle from a static thermal expansion of the piezoelectric element.
High pressure, which is generated in a pressure chamber, also referred to as a rail, by a high pressure fuel pump for example, prevails inside the control valve. The pressure generated by this high pressure fuel pump is referred to as rail pressure. In order to position the control valve accurately and thus implement a desired injection, a control voltage setpoint is required for the piezoelectric element. This control voltage setpoint is formed as a function of pressure. This voltage setpoint is additionally corrected as a function of a temperature of the piezoelectric element by using a multiplier.
However, in this method the control voltage characteristic curve determined is not applicable equally to all piezoelectric elements and all injectors. The reasons for the deviations occurring here lie first in the scattering of the stroke capability of the piezoelectric elements, and second in the mechanical tolerances of the injector components. The calculation of the voltage setpoint for determining the control voltage characteristic curve is not possible with the present method, due to specific correction values of the piezoelectric elements and/or the injectors which have not been taken into account.